Electric motors can generate considerable heat, thereby making motor cooling difficult, especially if the electric motor in question is used as the traction motor of an electric or hybrid vehicle where size and weight constraints are coupled with the need for high motor output. In addition to being efficient, the means used to cool the motor must be capable of operating in a variety of environments since the motor in a vehicle is typically subjected to a wide range of ambient temperatures, humidity levels and dust/dirt levels. Lastly, in order to avoid excessive wear due to differential thermal expansion, it is important to cool the internal motor components, such as the rotor, as well as the outer motor components, such as the casing and stator.
A variety of approaches have been taken to meet the cooling demands placed on a vehicle's electric motor. For example, U.S. Pat. No. 6,191,511 discloses using a closed loop, liquid cooling circuit to try and achieve a temperature balance within the motor, the cooling circuit passing the coolant through both the stator and a hollow rotor shaft. Within the hollow rotor shaft is a stationary injection tube, the injection tube fixed to the stator flange. The coolant is pumped through the injection tube to the end of the rotor shaft where it is driven back between the injection tube and the hollow rotor. The coolant then passes through a cylindrical cooling chamber extending over the length and periphery of the stator before cooling the stator structure and being returned to the injection tube.
U.S. Pat. No. 6,329,731 discloses a liquid cooled electric motor in which one of the main elements of the planetary gear drives the displacement pump of the cooling circuit. The coolant is driven through a stationary tube about which the hollow rotor shaft rotates. The coolant then passes between the stationary tube and the hollow rotor shaft before passing through a radiator incorporated into the motor and planetary gear casing.
U.S. Pat. No. 7,156,195 discloses an electric motor in which the liquid coolant is collected within the reduction gear case, not the motor case, thus avoiding deterioration and alteration of the motor magnets. The coolant from the reservoir is pumped through the end of a passage in the drive shaft where it flows toward the motor. Part of the coolant is sprayed onto the reduction gears while the rest of the coolant is pumped between the drive shaft and the reduction gear shaft and the motor output shaft.
Co-assigned U.S. Pat. Nos. 7,489,057 and 7,579,725 disclose a cooling system and method of use, respectively, utilizing a hollow rotor shaft. A coolant feed tube, rigidly attached to the hollow rotor shaft, pumps coolant into the rotor while rotating at the same rate as the rotor shaft. When the coolant exits the end of the feed tube it flows against the inside surface of the closed end of the rotor shaft where it is forced to change direction and flow back through the space between the outer surface of the feed tube and the inner surface of the hollow rotor shaft. After passing through the rotor shaft, the coolant is expelled through the open end of the rotor.
While the prior art discloses a number of techniques for cooling an electric motor, specifically the rotor of an electric motor, a robust, reliable, high efficiency cooling system that is straightforward and cost effective to manufacture and install is desired. The present invention provides such a rotor and cooling assembly.